This invention relates generally to plastic transparencies, and, more particularly, to transparencies formed of a multi-axially oriented (e.g., biaxially-oriented) plastic sheet material.
Plastic transparencies of this type are of particular use as airplane windows, especially when manufactured of stretch-modified polymethyl methacrylate meeting MIL-P-25690. Suitable methods and apparatus for forming such a stretched acrylic plastic material are described in U.S. Pat. Nos. 2,912,716 (Frownfelter et al), 2,918,696 (Bottoms et al), 3,596,311 (Salmon), and 3,632,841 (Fortin).
One drawback to the use of stretched acrylic as airplane windows is that cracks will frequently develop in the windows. These fractures ordinarily begin as crazes in the peripheral edge surface of the window and propagate inwardly after a period of use. The fractures are usually oriented in the plane of the transparencies, because of the anisotropic nature of the multi-axially oriented material. It has been observed that these in-plane fractures tend to occur much sooner in transparencies exposed to high humidity environments.
Plastic airplane windows of this type are customarily mounted using edge seals that serve to both eliminate pressure leaks and conceal edge cracks. For example, passenger airline cabin windows commonly use a rubber gasket around their peripheries, and cockpit windows and windshields, which are usually laminates of stretched acrylic and glass or other material, commonly used edge sealants formed of silicone rubber or polysulfide material. These sealants are opaque and are usually applied like putty, in thicknesses of about 0.060 to 1.100 inches.
Efforts have been made in the past to reduce these in-plane fractures, but apparently without success. In one known effort, abrasion-resistant coating materials based on polysiloxane chemistry were evaluated as edge sealants for cabin windows. However, no significant reduction in the occurrence of peripheral edge cracking in the windows was observed.
It therefore will be appreciated that there is a definite need for an improved plastic transparency for particular use as an airplane window, having an improved resistance to cracking in its peripheral edge surface, especially in a high humidity or variable humidity environment. The present invention fulfills this need.